B cells localize and proliferate primarily in which site?

• A. Germinal centers of spleen
• B. Paracortex of lymph node
• C. Medullary cords of the lymph node
• D. Follicles of lymph nodes

Answer: (D)

B cells reside in the B cell zones of secondary lymphoid organs, particularly in the follicles of lymph nodes. When an antigen is encountered, germinal centers form within these follicles and become the main sites where B cells proliferate, undergo somatic hypermutation, and class-switch recombination. That makes the follicles of lymph nodes the primary location for B cell expansion in this context. The paracortex is the T cell zone, medullary cords mainly house plasma cells, and while germinal centers of the spleen are also B cell proliferative sites, the question focuses on the lymph node environment, so the follicles are the correct site.